I squeezed Siege Towers in today - they rattle in from the edges with different sorts of cannons mounted on top. They also serve as defensive shields for any critters behind them - you'll have to fire over them to reach any hidden monsters, or blow them to bits. They are multi-part, so you can blow off the wheels or sections of the tower, or just the cannons on top. Hit 'em hard enough if they are on a slope, and you can tip them over, too! A powderkeg works nicely to blow out the wheels.